The present invention relates to bearings, and more particularly to bearings which are electrically insulated.
It is long been recognized that parasitic currents can flow through the rotor shaft of dynamoelectric machines. With some designs, --these shaft currents--may arise from undesired phenomena such as inductive imbalances, circulating currents, or the like.
Frequently the only electrical path between the stator and rotor of such a machine is through the bearing or bearings which support the rotor in the stator. As such bearings are conventionally made of conductive metal, which enables current to flow through the bearings. When this occurs the results are highly desctructive to the bearing surfaces. In particular, the flow of electric current gives rise to erosion of the bearing surfaces. As it is usually desired that the bearing surfaces be as smooth as possible, this deterioration results in a corresponding deterioration of bearing properties, and shortens the life of the bearing, sometimes making it necessary to remove the machine from operation and repair or rebuild it.
This phenemonon has been known for many years, and numerous attempts have been made to construct the bearings which are rugged and serviceable, and yet relatively economical. In past years, efforts were made to produce an insulating effect by using bearings comprised of concentric inner and outer members, between which were disposed insulating materials such as impregnated fabric, plastic, cork, and the like.
More recently efforts have been made to incorporate more modern materials. For example, U.S. Pat. No. 3,924,906-Kitaoka teaches a journal bearing for a dynamoelectric machine which is surrounded by another layer of an insulating ceramic material, which may be applied in molten form. The thin ceramic layer is then impregnated with a curable liquid insulating material, such as varnish, both to prevent contamination of the ceramic material and to effect some lubricity so that the bearing may be moved more easily, and reduce the liklihood of chipping or damage to the insulator surface. It is been found, however, that the insulating surface thus produced is susceptible of injury before and during assembly into the machine, and that maintaining the outside of the coated assembly to a precise tolerance presents additional difficulties. Accordingly, it is object of present invention to provide an improved plain bearing which overcomes the afore-mentioned difficulties.
Another object of present invention is to provide an improved insulated plain bearing wherein the insulation layer is not susceptible of injury.